Is there a advantage to having a frame over a unibody?

You can lift the body off the frame, build your suspension with the engine and everything in the car and then set everything down on top of it. Always pros and cons in everything.

Unibody is lighter in stock form, once you go full tube chassis that advantage would be gone.

Putting frame connectors in a unibody goes a long way in strengthening the frame to reduce flex and improve weight transfer. Either style needs to be topped off with at leat a 6-8 point roll bar for even better efficiency. I'm just guessing, but with a unibody, you can add reinforcements only where it's needed most to save weight.